Catalytic hydrogen evolution from a covalently linked dicobaloxime

Abstract

A dicobaloxime in which monomeric Co(III) units are linked by an octamethylene bis(glyoxime) catalyzes the reduction of protons from p-toluenesulfonic acid as evidenced by electrocatalytic waves at −0.4 V vs. the saturated calomel electrode (SCE) in acetonitrile solutions. Rates of hydrogen evolution were determined from catalytic current peak heights (k app ¼ 1100 AE 70 M −1 s −1 ). Electrochemical experiments reveal no significant enhancement in the rate of H 2 evolution from that of a monomeric analogue: The experimental rate law is first order in catalyst and acid consistent with previous findings for similar mononuclear cobaloximes. Our work suggests that H 2 evolution likely occurs by protonation of reductively generated Co II H rather than homolysis of two Co III H units.hydrogen evolving catalysts | solar fuel E fficient catalytic reduction of protons to dihydrogen is a requirement for one half of a functional solar water splitting system (1, 2). Progress has been made recently in identifying catalysts capable of producing hydrogen from acidic media using either small molecule mimics of hydrogenase active sites or other synthetic systems (3-11).Our work has focused on difluoroboryl-bridged Co II -diglyoxime complexes that catalyze hydrogen evolution at low overpotentials (12)(13)(14)(15). With these complexes, hydrogen evolution is initiated upon reduction to Co I , which then reacts with a proton donor to form a Co III -hydride (Co III H). The Co III H intermediate can either undergo subsequent protonation to release H 2 and a Co III species that is reduced to regenerate the catalyst (heterolytic route) or react with another Co III H to eliminate H 2 by a homolytic route. Alternatively, Co III H can be reduced further to form Co II H, which can react via similar heterolytic or homolytic routes (16). Digital simulations of electrocatalytic waves performed by Hu et al. (17) indicated that the bimolecular homolytic Co III H route is the dominant pathway for hydrogen evolution. Our analysis of electron transfer rates in the catalytic cycle confirmed that the barriers to H 2 evolution associated with this pathway are more favorable than that for protonation of Co III H (17, 18). Recent work in our group examining the mechanism of hydrogen evolution from a photogenerated hydridocobaloxime suggests that hydrogen evolution via protonation of Co II H is favored under certain conditions (Co III H in low concentrations with reducing equivalents in excess) (16). Computational studies reported by 20) and Muckerman (21) suggest that Co II H protonation is a very favorable reaction pathway and that relative fluxes through the homolytic Co III H and heterolytic Co II H channels depend on experimental conditions (relative concentrations of acid and catalyst).In the low-barrier homolytic mechanism, two Co III H species must diffuse together in solution in order to react and release H 2 . It follows that immobilization onto an electrode surface would disfavor this pathway (7,22,23). It is of interest in this context that...

Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen‐Evolution Reaction

“…However, they also stated that although simulation of the CV for a monometallic mechanism did not give satisfactory results, this possibility could not be ruled out. The activity comparison of a dicobaloxime and its monomeric analogue by Gray and co‐workers revealed no significant enhancement, which suggests that protonolysis rather than homolysis is responsible for catalysis 38. Therefore, a well‐defined bimetallic HER mechanism has not been recognized and is of great interest and fundamental importance.…”

Earth-abundant hydrogen evolution electrocatalysts

“…Savéant Moreover, electrochemical studies of a covalently linked dicobaloxime revealed that the rate of hydrogen evolution was not signicantly different from that of a monomeric analogue. 96 These experiments suggested that hydrogen evolution likely occurs by protonation of reductively generated Co II -H rather than homolysis of two Co III -H units.…”

Structural and electronic characterization of multi-electron reduced naphthalene (BIAN) cobaloximes

“…However, only marginal improvements over the most basic complexes have been realized. [34][35][36] The two most widely known and cited cobaloxime case studies are those that involve the dimethyl and diphenyl glyoxime ligands (specifically, [Co(dmgBF 2 ) 2 (MeCN) 2 ] and [Co(dpgBF 2 ) 2 (MeCN) 2 ], Fig. 1).…”